One known planar solid oxide fuel cell stack is described in European patent applications EP0668622A1 and EP0673074A1. These describe a planar solid oxide fuel cell stack comprising a plurality of solid oxide electrolyte members, each solid oxide electrolyte member having an anode electrode on a first surface and a cathode electrode on a second opposite surface to form a fuel cell. At least one interconnector is provided to connect the anode electrode of one fuel cell with the cathode electrode of an adjacent fuel cell such that the solid oxide fuel cells are connected in electrical series. The fuel cells are arranged in a plane on one or both sides of a hollow porous gas permeable support/distribution member, which supplies either fuel to the anode electrodes or oxidant to the cathode electrodes. The electrolytes of these solid oxide fuel cells are of the order of 1 μm to 50 μm, for example 10 μm.
The main problems with all solid oxide fuel cells are the high manufacturing costs, poor thermal expansion compliance and limited operational temperature range. The poor thermal expansion compliance of solid oxide fuel cells makes them intolerant to temperature differences and to thermal shocks.
A further problem with all solid oxide fuel cells is that the voltages are less than the Nernst value due to electrochemical and electrical losses in the fuel cells. These losses depend on the current density. The losses are due to activation in the electrodes, diffusion in the electrodes and porous gas permeable support member, electrolyte/electrode interfacial resistance, current collection in the electrodes and ionic resistance in the electrolyte. The activation losses dominate at low currents, the diffusion losses dominate at high currents and the resistive losses dominate at intermediate and high currents. Losses also arise due to current flow through the interconnectors.
Also leakage losses through the electrolyte, interconnectors, and around the periphery of the fuel cells gives rise to further losses. Voids and micro cracks through the components or leakage around the periphery of the components impair the electrochemical performance of the fuel cells in three respects. Firstly there is a loss of current by diffusion or leakage of fuel or oxygen. Secondly there is a loss of voltage due to reduced oxygen partial pressure difference across the electrolyte membrane of the fuel cell. Thirdly there is increased resistance in the anode electrodes due to the nickel electrodes becoming oxidised to nickel oxide.
Other losses may arise due to oxygen ion leakage currents in the interconnectors and in the support member, if these posses ionic conductivity. Further losses may arise due to spurious fuel cells formed between an anode of one fuel cell and the cathode of an adjacent fuel cell if the cathode of one fuel cell contacts the electrolyte of the adjacent fuel cell. The interconnector short-circuits the spurious fuel cell.
Another problem with solid oxide fuel cells is the chemical interaction between the substrate and the anode electrode.